The present invention relates to the liquefaction of a gas. More particularly, the present invention relates to a method and apparatus for the liquefaction of a lean natural gas having a pressure above atmospheric pressure.
Numerous reasons exist for the liquefaction of gases and particularly of natural gas. The primary reason for the liquefaction of natural gas is that the liquefaction reduces the volume of a gas by a factor of about 1/600, thereby making it possible to store and transport the liquefied gas in containers of more economical and practical design.
For example, when gas is transported by pipeline from the source of supply to a distant market, it is desirable to operate under a substantially constant high load factor. Often the capacity will exceed demand while at other times the demand may exceed the capacity of the line. In order to shave off the peaks where demand would exceed supply, it is desirable to store the gas when the supply exceeds demand, whereby peaks in demand can be met from material in storage. For this purpose it is desirable to provide for the storage of gas in a liquefied state and to vaporize the liquid as demand requires.
Liquefaction of natural gas is of even greater importance in making possible the transport of gas from a source of plentiful supply to a distant market, particularly when the source of supply cannot be directly joined with the market by pipeline. This is particularly true where transport must be made by ocean going craft. Ship transportation in the gaseous state would be uneconomical unless the gaseous materials were highly compressed, and then the system would not be economical because it would be impractical to provide containers of suitable strength and capacity.
In order to store and transport natural gas, the reduction of the natural gas to a liquefied state requires cooling to a temperature of about -240.degree. F. to -260.degree. F. at atmospheric pressure.
Numerous systems exist in the prior art for the liquefaction of natural gas or the like in which the gas is liquefied by passing it sequentially through a plurality of cooling stages, to cool the gas to successively lower temperatures until the liquefaction temperature is reached. In this instance, cooling is generally accomplished by indirect heat exchange with one or more expanded refrigerants such as propane, propylene, ethane, ethylene, and methane. Once the gas has been liquefied at the feed gas pressure, the gas is expanded to atmospheric pressure by passing the liquefied gas sequentially through a plurality of expansion stages. During the course of the expansion, the gas is further cooled to storage or transport temperature and its pressure reduced to atmospheric pressure, and significant volumes of the gas are flashed. The flashed vapors from the expansion stages are generally collected, compressed to the pressure of the feed gas and then combined with the feed gas.
The natural gas feed to such systems generally contains small amounts of nitrogen which is desirably removed from the liquefied gas prior to storage or transport. Accordingly, it is common practice to remove the nitrogen by passing the liquefied gas through a nitrogen removal column or the like to vaporize the nitrogen and a portion of the methane. The nitrogen-containing gas thus removed will usually contain sufficient methane to make it useful as a fuel. Consequently, such nitrogen-gas is used as a fuel to operate liquefaction plant equipment, such as compressors, etc. Conventionally, the amount fuel gas removed is controlled indirectly by determining the composition or BTU value of the fuel gas and maintaining a predetermined composition. This technique, thus, fails to consider the fuel needs of the plant. Accordingly, if the feed gas flow to the plant drops below normal and, therefore, less fuel is needed to operate the plant, the amount of fuel withdrawn will often be greater than needed and the excess must be disposed of, usually by flaring. Conversely, should the feed gas flow increase and the fuel demands increase, insufficient fuel gas will often be withdrawn and the deficit must be made up from other sources, such as by using part of the high quality gas being processed. Obviously, such practices are uneconomical and wasteful.